The preparation of ethylbenzene by the reaction of benzene with ethylene in the presence of aluminum chloride and hydrogen chloride is well known. The aluminum chloride and hydrogen chloride react with alkylated benzenes in the reaction mixture to form complex, hereafter referred to as aluminum chloride complex, or simply, complex. Much of this complex can be removed by merely allowing it to settle out. However, all the complex cannot be isolated from the reaction mixture because it is partially soluble, and therefore, remains in the process stream coming from the settler. Some complex also remains in the process stream due to entrainment (dispersed as tiny droplets). Removal of this remaining dissolved and entrained complex has been accomplished by several methods. Washing the reaction mixture with water is the common industrial method of removing this remaining aluminum chloride complex. However, this is undesirable because water is introduced into the stream, which necessitates a costly drying process. This drying is necessary in commercial operations using recycle, because a substantial portion of the reactor stream is unreacted benzene, which must be dry before it can be recycled to the reactor.
U.S. Pat. No. 3,342,885 teaches the formation of an aluminum chloride complex precipitate in the reaction mixture of some reactions catalyzed by AlCl.sub.3 by the addition of certain oxygen-containing agents, and then filtering this reaction mixture to remove the precipitated aluminum chloride. Removal by filtration involves batch-type process operations in commercial ethylbenzene plants, which even if a suitable precipitates would form, would be undesirable compared to the accepted continuous method of water-washing.
An anhydrous, economically feasible method wherein the ethylbenzene product could be substantially separated from the aluminum chloride complex remaining in the stream following a settling separation, would be highly advantageous. Distillation was investigated as an anhydrous means of separating the ethylbenzene product from the aluminum chloride complex. In these investigations it was discovered that aluminum chloride complex, itself, is volatile, and highly efficient fractional distillation is required to separate the ethylbenzene from the complex. Because the method of fractional distillation would be expensive due to the large amount of reflux required, a method in which the separation could be attained by a flash distillation with little or no reflux would be more advantageous.